This invention relates to an improved process for coating metal surfaces and more particularly relates to improvements in the process for applying a protective paint base coating to metallic surfaces such as zinc, iron and aluminum. The need for applying protective coatings to metal surfaces has long been recognized in the art. These coatings are normally referred to as "conversion coatings" because the metal surface is converted from a chemically active one readily susceptible to oxidation to a surface which is relatively inactive and resistant to oxidation. One of the most important common uses of these conversion coatings is as a base for subsequent painting. Under these circumstances, the user will look to the adhesion of the paint to the metallic surface, as well as the resistance of the painted surface to humidity, salt-spray and similar tests in order to determine the corrosion resistance of the painted article.
In the past, difficulties have often been encountered in developing a method for treating metal surfaces which produces an end product which, when painted, exhibits satisfactory adhesion and corrosion characteristics in all respects. In some instances, it has been possible to obtain satisfactory results by subjecting the metal surface to a series of chemical treatments. The most commonly used process of this type involves a first treatment of the cleaned surface with a conversion coating solution which, for example, may produce zinc or iron phosphate coatings on steel, zinc phosphates or complex oxides on galvanized steel or chromic oxide or phosphate coatings on aluminum followed by a second treatment with a dilute hexavalent chromium-containing composition with an intervening water rinse. However, multiple-stage treating involves additional personnel, inter-stage contamination problems, and extended line length when compared to a single-application process as in the present invention. Because of the potential for inter-stage contamination due to drag-out, rinsing steps must be added which increase the consumption of water and add to disposal and pollution problems. Furthermore, the use of compositions containing hexavalent chromium compounds is environmentally objectionable and should be avoided if possible.
The present invention provides a single-application method for the treatment of bare metal surfaces for the purposes of forming a corrosion-resistant coating for paint base applications thereon. It has been found that, if the metal is contacted with an aqueous working composition containing a melamine-formaldehyde resin and a vegetable tannin, and the thus-treated surface is subsequently dried, a coating is obtained which exhibits characteristics comparable to a conventional conversion coating. If either of the components is omitted from the composition, the conversion coating is deficient in one or more respects. As mentioned, this single-application process has obvious advantages over prior art methods and employing two or more stages to obtain a conversion coating having comparable properties.
U.S. Pat. No. 3,397,077 teaches the use of an aqueous system containing an inorganic oxyacid (such as phosphoric acid) and an organic resin-forming material (e.g. based upon a melamine-formaldehyde reaction product) but makes no suggestion of the advantages of including a vegetable tannin in the composition.